Drill stem tests are conducted primarily to determine whether a fluid bearing formation penetrated by the well bore will produce oil or gas in sufficient quantities to justify completing the well in that formation. To obtain this information, the formation is relieved of the hydrostatic pressure of the drilling fluid in the well bore sufficiently to allow the fluids in the formation to flow into the well bore and up the drill pipe under substantially the same conditions that would exist after the well is completed. Measurements are made of the pressure in the well bore adjacent the formation, while the formation is flowing and while it is shut in. It is also helpful in the evaluation of the formation to obtain a sample of the produced fluid at the pressure and temperature of the producing formation. This is the function of the bottom hole sampler to which this invention relates.
Most bottom hole samplers include a tubular member that is connected into the drill string and forms part of the passageway through which the formation fluid travels as it moves up the drill pipe. The sampler is usually located in the drill string as close to the producing formation as practical. At the end of the test, valves located at opposite ends of the tubular member are closed trapping the formation fluid in the sampler at or about the temperature and pressure of the fluid in the formation.
In the past, both slide valves and ball valves have been used in samplers. Ball valves are generally preferred because, when open, they do not reduce the opening through the sampler below that of the drill pipe. For examples of these prior art samplers using both slide and ball valves, see U.S. Pat. Nos. 3,308,887 and 4,063,593.
Ball valve type samplers in the past have been prone to malfunction due to solids, such as cuttings, wall cake, barites or the like, collecting in the somewhat complicated parts used to operate the ball valves. In many cases, if one of the ball valves will not close for any reason, the other will not either, since they are both closed by the same mechanism. This is not a critical situation as far as the sampler is concerned since if one doesn't close, the sampler cannot function properly anyway, but it is important when the sampler is acting as a safety valve.